1. Field of the Invention
The present invention relates to a liquid fuel mixing apparatus for uniformly maintaining the concentration of a liquid fuel to be supplied to a direct liquid feed fuel cell.
2. Discussion of the Background
Direct liquid feed fuel cells generate electricity through an electrochemical reaction between an organic compound fuel such as methanol or ethanol and an oxidizing agent. Direct liquid feed fuel cells have very high energy and power density. Because direct liquid feed fuel cells directly use a liquid fuel such as methanol, they do not require peripheral devices such as fuel reformers, and fuel storage and supply are simplified.
As shown in FIG. 1, a direct liquid feed fuel cell may include an electrolyte membrane 1 interposed between an anode 2 and a cathode 3. The anode 2 and the cathode 3 may include fuel diffusion layers 22 and 32 for fuel supply and diffusion, catalyst layers 21 and 31 for the redox reaction of fuel, and electrode support layers 23 and 33. A catalyst for the electrode reaction may be a noble metal catalyst with good catalytic characteristics even at low temperatures, such as platinum. A transition metal-alloy catalyst derived from ruthenium, rhodium, osmium, nickel, or the like may be used to prevent catalyst poisoning by reaction byproducts such as carbon monoxide. The electrode support layers 23 and 33 may be carbon paper, carbon cloth, or the like, and are water-proofed to facilitate the supply of fuel and the discharge of reaction products. The electrolyte membrane 1 may be a polymer membrane about 50 to about 200 μm thick. A hydrogen ion exchange membrane that contains moisture and has ionic conductivity may be used as the electrolyte membrane 1.
Direct methanol fuel cells (DMFCs) are a type of direct liquid feed fuel cells that use a mixed fuel of methanol and water. DMFC electrode reactions include a fuel oxidation (anode reaction) and an oxygen reduction in the presence of protons (cathode reaction). These reactions are summarized as follows:
Anode reaction: CH3OH+H2O→CO2+6H++6 e−
Cathode reaction: 3/2 O2+6 H++6 e−→3H2O
Overall reaction: CH3OH+3/2 O2→2 H2O+CO2 
At the anode 2 at which fuel oxidation occurs, methanol reacts with water to produce carbon dioxide, protons, and electrons. The generated protons travel to the cathode 3 through the electrolyte membrane 1. At the cathode 3 at which oxygen reduction occurs, the protons, electrons supplied from an external circuit, and oxygen react to produce water. In the overall reaction of DMFCs, water and carbon dioxide are produced through the reaction of methanol and oxygen. The reaction of 1 mole of methanol with oxygen produces 2 moles of water.
Usually, the liquid fuel used in DMFCs is not pure methanol but is a mixture of methanol and water which is generated or stored in a system. When a high concentration fuel is used, electricity generation performance may be considerably decreased due to fuel crossover. Fuel crossover is the phenomenon of fuel permeation through an electrolyte membrane (hydrogen ion exchange membrane). Dilute methanol with a concentration of about 0.5 to about 2 M (2 to 8 vol. %) may be used to prevent fuel crossover.
However, in currently available DMFC systems that include a fuel tank to store highly concentrated or pure methanol, it is difficult to supply to an anode a mixture of methanol from the fuel tank and water recovered from the systems or supplied from a separate water tank with a uniform concentration.